1. Field of the Invention
The present invention relates to a process and device for the formation of a multi-ply and/or multi-layer fiber web, and more particularly to a process and device for the formation of a multi-ply and/or multi-layer paper or cardboard web.
2. Description of the Related Art
The conservative use of raw material resources, in particular for economic and ecological reasons, impacts paper production in the form of ever-lower area-specific weights of paper webs, as well as in the partial substitution of the pulp material with fillers. This constitutes a challenge not only for the paper producer, but also for the manufacturer of paper production machines. Paper quality and the productivity of the machines must not only be maintained in spite of these measures, but are supposed to be further improved. In this context, the so-called multi-ply technology is increasingly deployed, in which, in comparison to conventional multi-layer sheet formation, the web is built from individual layers with a very low area-specific weight. In this process, various suspension flows exiting from the headbox are simultaneously drained off in one and the same web-formation zone. Accordingly, various types of raw materials can be used to structure the web. If low cost raw materials are used, the quality is to be maintained. With the same type of raw material, the question of how to improve the quality becomes relevant. In all of this, stability, optical quality, and further processability of the end product play a critical role. For example, it may be desirable for graphic paper types, for economic reasons and reasons of printing technology, to have a higher filler content in the external layers and a lesser filler content in the middle layer. The higher filler content in the external layers produces paper that is better to print on and has improved opacity. Even when using multi-ply technology in combination with a multi-layer headbox and a high filler content in the partial suspension flows, which form the external layers, results are still unsatisfactory, the cause of which resides mainly in the low rate of filler retention. Accordingly, the suspension flows of each headbox, that form the external layers, require a much increased filler content. Due to the low retention rate, the filler load on the water strainer cycle is high. Due to the external layers being washed out as a result of the prevailing drainage conditions, only a limited enrichment of the external layers of the paper with ashes results.
It is the objective of the invention to develop an improved process and an improved device of the aforementioned type, in which the aforementioned disadvantages are eliminated, making it possible to increase the enrichment of the external layers of the pulp material web with fillers in an economic manner and beyond the limits that have existed so far.
The present invention provides a process, and a device in which at least one of the two external layers and/or plies of the pulp material web is produced using pulp material onto the wetted fiber surfaces of which, at least partially, an additive, preferably a filler, has been deposited, or the fibers of which have been loaded with at least one additive, particularly a filler.
In this context, the concept of xe2x80x9cwetted fiber surfacesxe2x80x9d may include all wetted surfaces of the individual fibers. This covers in particular also the cases where the fibers are loaded with an additive or filler, both on their external sides and in their interior spaces (lumen).
Due to this special form of loading, the retention rate in the external layers is drastically increased, which is due to the circumstance that the additives, here in particular fillers, are bonded tightly to the exterior fiber surface and to some extent also to the interior fiber surface. The association between the fillers and the fiber surface is relatively strong. The result is a high filler retention rate in the external layers or plies of the paper with a low load on the water strainer and a relatively low suspension filler concentration in the external layers or plies in the headbox. If required, the filler content in the layers or plies can even be further increased in conventional ways. The invention cannot only be used when forming a fiber web from pulp material suspensions layered in the direction of the z-axis, i.e., in thickness direction, with different additive or filler contents, but also when forming a fiber web from pulp material suspensions layered in the direction of the z-axis with identical filler contents. In addition, the invention cannot only be used in the preferred multi-ply sheet formation, but also in conventional multi-ply sheet formation. It is essential that so-called xe2x80x9cfiber loadingxe2x80x9d be used for the materials in the external layers, which has been described in U.S. Pat. No. 5,223,090. In the process that is described hereby, the entire material is treated accordingly. Also, the procedure of loading additive or filler into an external or internal ply or layer may follow the process as described in U.S. Pat. No. 5,223,090, the content thereof is incorporated into this patent application by reference.
In the process, an additive is deposited on the external surfaces of the hollow fibers, and it is also preferably deposited in the interior hollow areas and in the walls of the hollow fibers. The additive is thus deposited on the xe2x80x9cwettedxe2x80x9d fiber surface, i.e., both on the exterior surface and on the interior hollow spaces, as well as into the walls of the hollow fibers.
In one embodiment of the invention, there is formed a fiber material web that features at least two and in particular three plies or layers, both or only one of the external plies or layers of the pulp material web are produced using pulp material, on the wetted fiber surfaces of which, at least partially, at least one additive has been deposited. Accordingly, both plies or layers or only one of the external plies or layers of the fiber material web can be produced using fiber material, the hollow fibers of which have been loaded at least partially with at least one additive, which preferably has, again, been deposited both on the external surfaces of the fibers and in their interior spaces.
The additive may preferably include at least one filler.
For the formation of at least a two-ply or two-layer and in particular of an at least three-ply or three-layer fiber material web, at least one of the two external plies or layers is produced with a higher additive or filler content than at least one intermediate ply or layer.
In one embodiment of the invention, calcium carbonate is deposited, as an additive or filler, on the wetted fiber surfaces.
A preferred embodiment of the invention is characterized by the deposition of calcium carbonate, a substance containing calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) to the wet disintegrated fiber material in such a manner that at least part of it associates with the water that is contained in the fiber material. The fiber material treated in this manner is loaded with carbon monoxide in such a way that the deposition of calcium carbonate on the wetted fiber surfaces occurs as the result of a corresponding chemical reaction. This does not mean that by adding CaO and/or Ca(OH)2, water of the fiber material suspension must be chemically transformed.
In this process, the substance containing calcium oxide and/or calcium hydroxide will be added at a rate of approximately 0.1% to 50% by weight with respect to the dry weight of the pulp material in question. The pulp material is preferably loaded with carbon monoxide in a closed container that is appropriately pressurized with carbon dioxide gas.
It is advantageous if the pulp material is loaded with carbon dioxide during a mixing process, in which it is exposed to increased shear forces. In this process, the mixing procedure should be performed with a shearing effect that is at least so high as to result in an energy input of at least approx. 10 to 50 W-hr per kg of fiber on the basis of dry weight.
According to this invention it is also possible to produce only one of the two plies or layers of the fiber material web, in order to form a two-ply or two-layer fiber material web. This is accomplished by using pulp material, on the wetted fiber surfaces of which an additive has at least partially been deposited in the manner described above.
Regarding pulp material, it is advantageous if wood pulp dissolved in a pulper or dissolved waste paper with additives are used, or xe2x80x9cnever dried pulpxe2x80x9d is used. Both applications result in advantages, with regard to process technology (including short processing times), control technology (including good controllability), and costs (including cost reduction potentials), depending on the application.
The invention is also characterized by one material suspension feed that is provisioned for the formation of an external ply and/or external layer of the pulp web, which can be loaded with pulp material, on whose fiber surfaces at least one additive has been at least partially deposited.
In one embodiment of the invention, the device includes at least one multi-layer headbox, which, (in order to form two plies, or in order to form two external plies, or in order to form two external plies and at least one internal ply located between them) features at least two material suspension feeds in which only one or two of the material suspension feeds, serve to form the two external plies, which are loaded with pulp material, and on whose fiber surfaces at least one additive has been at least partially deposited.
A further embodiment of the invention provides for the formation of two layers, or two external layers and at least one internal layer that is located between them. The invention includes at least two headboxes, which each feature at least one material suspension feed, and in which at least one headbox is instrumental in the formation of the two external layers, and can be loaded with pulp material, in which the fiber surfaces have an additive which has been at least partially deposited.
The additive or filler, again, may be calcium carbonate.
Another embodiment of the invention provides a device which includes at least one multi-layer headbox, in order for form two plies, featuring two material suspension feeds, and in which only one of these two material suspension feeds is loaded with pulp material, on whose fiber surfaces an additive has been at least partially deposited, in the manner described above.
A further embodiment of the invention includes at least two headboxes, for the purpose of forming several layers, of which at least one is loaded with pulp material, on the wetted surfaces of the fibers therein at least one additive has at least partially been deposited, in the manner described before.
In addition, the material suspension may include wood pulp that has been dissolved in a pulper, or dissolved waste paper with additives, or xe2x80x9cnever dried pulpxe2x80x9d.